This is a continuation-in-part application of International Application PCT/DK92/00310 with an international filing date of Oct. 26, 1992, said International Application being based upon Danish Application 1922/91.
The present invention relates to a method for manufacturing cement.
Hitherto, in manufacturing cement it has been common practice that the finish-burnt and cooled clinker material is mixed with gypsum and possibly other additives and subsequently finish-ground in a grinding plant.
The added gypsum usually contains water which is required to be removed to avoid setting of the cement, and in grinding plants comprising a tube mill, the dehydration and drying of the gypsum often take place in the mill, in which the temperature is relatively high.
Tube mills are, however, relatively inefficient in that they are energy-demanding, and in fact the tendency today is that roller mills are used instead for finish-grinding of cement due to their higher efficiency.
However, the temperature in roller mills is rarely so high that a sufficient drying of the added gypsum is obtained, which has an adverse influence on the cement properties.
This problem has been dealt with in the past by adding gypsum which, before the addition, is dehydrated and dried in a separate drying installation, but this increases the preliminary expenses as well as the operating costs.
Another proposed solution to the problem consists of heating the material during the grinding by means of gas from a heat generator or by means of excess air from a clinker cooler, but as the retention time in, e.g., a roller mill is very short, the possibilities of obtaining a sufficient drying of the gypsum are very small.
From e.g. DD-201137, SU-1313821 and GB-1551269 it is known to introduce material components containing gypsum or the like into the zone between the kiln and the clinker cooler or into the first part of the clinker cooler, where the temperature typically is in the range from 800.degree. to 1400.degree. C.
The disadvantage of introducing the gypsum into or before the first and thereby the hottest part of the clinker cooler is that the gypsum is converted into insoluble anhydride which, when the finish-ground cement is used for making cement paste, mortar or concrete, has lost its desired effect which is to delay the setting of the cement.